1. The Digestive System
2011

2. What do humans need to live?
Humans make energy using:
food
oxygen
Humans build bodies using:
food for raw materials
amino acids, sugars, fats, nucleotides
ATP energy for synthesis
food
ATP O2

3. Nutritional requirements
Humans are heterotrophs
need to take in food
Why? fulfills 3 needs…
fuel = chemical energy for production of ATP
raw materials = carbon source for synthesis
essential nutrients = animals cannot make
elements (N, P, K, Fe, Na, K, Ca...)

5. Getting & Using Food Ingest Digest Absorb Eliminate taking in food
mechanical digestion
breaking up food into smaller pieces
chemical digestion
breaking down food into molecules small enough to be absorbed into cells
enzymes
Absorb
absorb across cell membrane
diffusion
active transport
Eliminate
undigested extracellular material passes out of digestive system

6. Human digestive system Alimentary Canal
After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested.
Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours.
In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

7. Common processes & structures
Movement & Control
peristalsis
push food along by rhythmic waves of smooth muscle contraction in walls of digestive system
sphincters
muscular ring-like valves, regulate the passage of material between sections of digestive system
Accessory glands
salivary glands, pancreas, liver & gall bladder
secrete digestive juices (enzymes & fluid)

8. Swallowing (& not choking)
Epiglottis
problem: breathe & swallow through same orifice
flap of cartilage
closes trachea (windpipe) when swallowing
food travels down esophagus
Esophagus
move food along to stomach by peristalsis

9. Ingestion Mouth mechanical digestion chemical digestion teeth saliva
breaking up food
chemical digestion
saliva
amylase
enzyme digests starch
mucin
slippery protein (mucus)
protects soft lining of digestive system
lubricates food for easier swallowing
buffers
neutralizes acid to prevent tooth decay
anti-bacterial chemicals
kill bacteria that enter mouth with food

10. mouth
break up food
moisten food
digest starch
kill germs

11. Stomach Functions food storage disinfect food chemical digestion
can stretch to fit ~2L food
disinfect food
HCl = pH 2
kills bacteria
breaks apart cells
chemical digestion
pepsin
enzyme breaks down proteins
secreted as pepsinogen
activated by HCl
Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days.
Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Heliobacter pylori.
Ulcers are often treated with antibiotics.
Pepsin is secreted in an inactive form, called pepsinogen by specialized chief cells in gastric pits.
Parietal cells, also in the pits, secrete hydrochloric acid which converts pepsinogen to the active pepsin only when both reach the lumen of the stomach, minimizing self-digestion.
Also, in a positive-feedback system, activated pepsin can activate more pepsinogen molecules.
But the stomach is made out of protein!
What stops the stomach from digesting itself?
mucus secreted by stomach cells protects stomach lining

12. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
store food
break up food
digest proteins
cardiac sphincter
pyloric sphincter

13. Ulcers Used to think ulcers were caused by stress
Coevolution of parasite & host
Ulcers
Free of H. pylori
Colonized by H. pylori
Used to think ulcers were caused by stress
tried to control with antacids
Now know ulcers caused by bacterial infection of stomach
Helicobacter pylori
now cure with antibiotics
inflammation of stomach
inflammation of esophagus
H. pylori
cell damaging proteins
(VacA)
inflammatory proteins
(CagA)
cytokines
helper T cells
neutrophil cells
white blood cells

14. Small intestine Function Structure
major organ of digestion & absorption
chemical digestion
digestive enzymes
absorption through lining
over 6 meters!
small intestine has huge surface area = 300m2 (~size of tennis court)
Structure
3 sections
duodenum = most digestion
jejunum = absorption of nutrients & water
ileum = absorption of nutrients & water
About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles.
As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient-rich broth known as acid chyme.
At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine.
A squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty.

15. Duodenum 1st section of small intestines
acid food from stomach mixes with digestive juices from accessory glands:
pancreas
liver
gall bladder

16. What stops pancreas from digesting itself
Ooooooh!
Zymogen!
Pancreas
Digestive enzymes
peptidases
pancreatic amylase
Pancreas song by Weird Al
Buffers
reduces acidity
alkaline solution rich in bicarbonate (HCO3-)
buffers acidity of material from stomach
small intestines

17. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
pancreas
produces enzymes to
digest proteins & starch

18. Liver Digestive System Functions produces bile
stored in gallbladder until needed
breaks up fats
act like detergents to breakup fats
Circulatory System Connection
bile contains colors from old red blood cells collected in liver =
iron in RBC rusts & makes feces brown

19. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins & starch

20. Digestive enzymes

21. Absorption by Small Intestines
Absorption through villi & microvilli
finger-like projections
increase surface area for absorption
Ooooh…
Structure-Function theme!

22. Absorption of Nutrients
Passive transport
fructose
Active (protein pumps) transport
pump amino acids, vitamins & glucose
against concentration gradients across intestinal cell membranes
allows intestine to absorb much higher proportion of nutrients in the intestine than would be possible with passive diffusion
worth the cost of ATP!
nutrients are valuable… grab all you can get!

23. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
small intestines
breakdown all foods
- proteins
- starch
- fats
- nucleic acids
absorb nutrients
pancreas
produces enzymes to
digest proteins & starch

24. Large intestines (colon)
Function
re-absorb water
use ~9 liters of water every day in digestive juices
> 90% of water reabsorbed
not enough water absorbed back to body
diarrhea
too much water absorbed back to body
constipation

25. Flora of large intestines
You’ve got company!
Flora of large intestines
Living in the large intestine is a rich flora of harmless, helpful bacteria
Escherichia coli (E. coli)
a favorite research organism
bacteria produce vitamins
vitamin K; biotin, folic acid & other B vitamins
generate gases
by-product of bacterial metabolism
methane, hydrogen sulfide
Folic acid: coenzyme needed for DNA & RNA synthesis and proper neural tube growth, may have role in cancer prevention
Biotin: coenzyme needed for Krebs cycle, fatty acid synthesis & gluconeogenesis

26. Rectum Last section of colon (large intestines) eliminate feces salts
undigested materials
extracellular waste
mainly cellulose from plants
roughage or fiber
salts
masses of bacteria
The study of the rabbit is fascinating, and from periods of quiet observation we learn some of the peculiarities of its life and habits. One of the most interesting of these is coprophagy. The word comes from the Greek kopros (dung) and phago (eating). This dung eating is not quite so revolting as it sounds at first, for the rabbit makes a special form of pellet which it takes directly from its anus. Coprophagy plays an important part in the digestive/nutritional process.
This practice involves ingestion of special soft fecal pellets which are excreted in the early morning hours. This is a significant practice in that the bacterial synthesis of certain B vitamins in the cecum are excreted at this time and if rabbits are prevented from this practice they will die from vitamin B deficiency within a rather short period of time.
The special soft pellets are produced at night or during periods of rest and are often called "nocturnal pellets" to distinguish them from the fecal pellets excreted at other times. The process has a distinct analogy with the chewing of the cud by ruminants.
Like the cow, rabbits are herbivorous and their diet contains a high proportion of crude fiber. The cellulose of the fiber has to be broken down before complete digestion and absorption can take place. The rabbit has a comparatively large caecum and colon to facilitate this. In order to obtain the maximum nutriment from its food the rabbit has developed the habit of coprophagy, passing certain of its intestinal contents through the system twice.
In addition to the improved nutrition, it is possible that the soft pellets fulfill a need to give greater bulk to the stomach contents. The rabbit's stomach and intestines are geared to bulk supplies and under some conditions the diet may lack bulk. The stomach has a comparatively poor muscular action and relies to a great extent on the pressure of successive meals to push the mass of food along the digestive tract.
The composition of the two types of pellets is interesting, the soft pellets having much more protein and less crude fiber. The process is controlled by adrenal glands.
appendix

27. mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
break up food
digest proteins
store food
liver
produces bile
- stored in gall bladder
break up fats
small intestines
breakdown food
- proteins
- starch
- fats
absorb nutrients
pancreas
produces enzymes to
digest proteins & carbs
large intestines
absorb water

28. Appendix
Vestigial organ

29. Hungry for Information?
Ask Questions!